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Re: [PATCH 09/11] pdp11 - example of a port not needing a speculation barrier


On July 27, 2018 3:27:49 PM GMT+02:00, Paul Koning <paulkoning@comcast.net> wrote:
>
>
>> On Jul 27, 2018, at 5:37 AM, Richard Earnshaw
><Richard.Earnshaw@arm.com> wrote:
>> 
>> 
>> This patch is intended as an example of all that is needed if the
>> target system doesn't support CPUs that have speculative execution.
>> I've chosen the pdp11 port on the basis that it's old enough that
>this
>> is likely to be true for all existing implementations and that there
>> is also little chance of that changing in future!
>> 
>> 	* config/pdp11/pdp11.c (TARGET_HAVE_SPECULATION_SAFE_VALUE):
>Redefine
>> 	to speculation_safe_value_not_needed.
>> ---
>> gcc/config/pdp11/pdp11.c | 3 +++
>> 1 file changed, 3 insertions(+)
>> 
>> <0009-pdp11-example-of-a-port-not-needing-a-speculation-ba.patch>
>
>Correct, no speculative instruction now, and I don't think any of the
>people constructing PDP11s (yes there are some) are going to be adding
>it.

It's not really about speculative instructions but about things like having a branch predictor and ways to recover from badly predicted ones and thus from wrongly speculatively executed regular instructions. 

Probably every pipelined CPU implementation nowadays has speculative execution. 

Richard. 

>Thanks Richard.
>
>	paul


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